Plant Chitinase Mutants as the Catalysts for Chitooligosaccharide Synthesis Using the Sugar Oxazoline Derivatives

Sugar oxazolines, (GlcNAc)_n-oxa (n = 2, 3, 4, and 5), were synthesized from a mixture of chitooligosaccharides, (GlcNAc)_n (n = 2, 3, 4, and 5), and utilized for synthesis of (GlcNAc)₇ with higher elicitor activity using plant chitinase mutants as the catalysts. From isothermal titration calorimetry, the binding affinity of (GlcNAc)₂-oxa toward an inactive mutant obtained from Arabidopsis thaliana GH18 chitinase was found to be higher than those of the other (GlcNAc)_n-oxa (n = 3, 4, and 5). To synthesize (GlcNAc)₇, the donor/acceptor substrates with different size combinations, (GlcNAc)₂-oxa/(GlcNAc)₅ (1), (GlcNAc)₃-oxa/(GlcNAc)₄ (2), (GlcNAc)₄-oxa/(GlcNAc)₃ (3), and (GlcNAc)₅-oxa/(GlcNAc)₂ (4), were incubated with hypertransglycosylating mutants of GH18 chitinases from A. thaliana and Cycas revoluta. The synthetic activities of these plant chitinase mutants were lower than that of a mutant of Bacillus circulans chitinase A1. Nevertheless, in the plant chitinase mutants, the synthetic efficiency of combination (1) was higher than those of the other combinations (2), (3), and (4), suggesting that the synthetic reaction is mostly dominated by the binding affinities of (GlcNAc)_n-oxa. In contrast, the Bacillus enzyme mutant with a different subsite arrangement synthesized (GlcNAc)₇ from combination (1) in the lowest efficiency. Donor/acceptor-size dependency of the enzymatic synthesis appeared to be strongly related to the subsite arrangement of the enzyme used as the catalyst. The A. thaliana chitinase mutant was found to be useful when combination (1) is employed for the substrates.

Development of chemical and chemo-enzymatic glycosylations

Glycosidic compounds are indispensable molecules in living systems. Biological phenomena such as cell wall formation, energy storage, and cell recognition strongly depend on the multi-functional characters of these substances. Development of highly regio- and stereoselective glycosylation reactions is necessary to provide sufficient amounts of specific compounds in basic research as well as for applications in industry. This review presents an overview of chemical and chemo-enzymatic glycosylations that have been developed during my forty-year academic career in the field of glyco-science. In the course of these studies, several new concepts such as "Direct Anomeric Activation", "Glyco-Process Chemistry" and "Glyco-Chemistry Cycles" have been established.

A common glycan structure on immunoglobulin G for enhancement of effector functions

Antibodies have been developed as therapeutic agents for the treatment of cancer, infection, and inflammation. In addition to binding activity toward the target, antibodies also exhibit effector-mediated activities through the interaction of the Fc glycan and the Fc receptors on immune cells. To identify the optimal glycan structures for individual antibodies with desired activity, we have developed an effective method to modify the Fc-glycan structures to a homogeneous glycoform. In this study, it was found that the biantennary N-glycan structure with two terminal alpha-2,6-linked sialic acids is a common and optimized structure for the enhancement of antibody-dependent cell-mediated cytotoxicity, complement-dependent cytotoxicity, and antiinflammatory activities.

The Amazing Transglycosylation Activity of Endo-β-N-acetylglucosaminidases

Major advances have been made in exploring the transglycosylation activity of endo-β-N-acetylglucosaminidases (ENGases) for synthetic purpose. The exploration of synthetic sugar oxazolines as donor substrates for the ENGase-catalyzed transglycosylation has expanded the substrate availability and significantly enhanced the overall transglycosylation efficiency. On the other hand, site-directed mutagenesis in combination with activity screening has led to the discovery of the first generation ENGase-based glycosynthases that can use highly active sugar oxazolines as substrates for transglycosylation but lack hydrolytic activity on the ground-state products. ENGases have shown amazing flexibility in transglycosylation and possess much broader substrate specificity than previously thought. Now the ENGase-based chemoenzymatic method has been extended to the synthesis of a range of complex carbohydrates, including homogeneous glycopeptides, glycoproteins carrying well-defined glycans, novel oligosaccharide clusters, unusually glycosylated natural products, and even polysaccharides. This article highlights recent advances related to ENGase-catalyzed transglycosylation with a focus on their synthetic potential.

Arthrobacter endo-beta-N-acetylglucosaminidase shows transglycosylation activity on complex-type N-glycan oxazolines: one-pot conversion of ribonuclease B to sialylated ribonuclease C

Asparagine-linked glycosylation is a major form of posttranslational modifications, which plays important roles in protein folding, intracellular signaling, and a number of other biological recognition events [1 ]. Glycoproteins are often characterized by their structural micro-heterogeneity where different glycoforms have the same polypeptide backbone but differ in the pendant oligosaccharides. Of particular interest are the findings that subtle difference in the attached glycans can have a significant impact on the biological functions of a given glycoprotein [2 , 3 ]. The urgent need of pure glycoforms for functional studies and biomedical applications has stimulated a great interest in exploring new methods for making homogeneous glycoproteins [4 ]. Major advances include the application of native chemical ligation and expressed protein ligation for constructing full-size glycoproteins [5 –7 ], chemoselective ligation to introduce homogeneous glycans [8 ], and the engineering of yeast glycosylation pathways to produce single glycoforms [9 ]. Yet another interesting advance in the field is the endoglycosidase-catalyzed transglycosylation for glycosylation engineering and glycoprotein synthesis [10 –16 ].